The present invention relates to the dead time reduction for multiband synthesizer frequency jumps. In particular, the present invention relates to the dead time reduction for multiband synthesizers allowing to generate output signals in at least two frequency bands.
Multiband synthesizer units are typically used in mobile phones. Here, the output signal of the multiband synthesizer is supplied to different mixer stages for sending and receiving signals in mobile phones, e.g., for the modulation of sending signals and the demodulation of received signals.
FIG. 11 shows a related frequency generation unit 200. Here, the object is to tune the frequency of a voltage-controlled oscillator 202 after frequency division thereof such that it is coincident to a frequency of a basic oscillator 204. As shown in FIG. 11, the basic oscillator 204 comprises a reference oscillator 206 that feeds a first programmable divider 208 to convert the frequency generated in the reference oscillator 206 into a control frequency for the operation of the voltage-controlled oscillator 202.
As also shown in FIG. 11, a second programmable divider 210 is provided to convert the frequency generated by the voltage-controlled oscillator 202 into a frequency suitable for comparison with the reference frequency of the basic oscillator 204. A phase detector 212 enables a comparison of the output signal of the second programmable divider 210 and the reference frequency. A detected phase error is then supplied to a loop filter 214 wherein an integration takes place. Using this integrated error signal the voltage-controlled oscillator 202 is controlled until the phase difference vanishes. Usually, the first programmable divider 208, the second programmable divider 210 and the phase detector 212 form the frequency synthesizer 216 of the frequency generation unit 200 of the PLL type.
FIG. 12 shows the embedding of this frequency generation unit 200 into a single band frequency generation device.
As shown in FIG. 12, the frequency generation unit 200 is connected to a control unit 218 provided for the operation and the programming of the frequency generation unit 200. This control unit 218 supplies different control signals and programming data signals to the frequency generation unit 200 either during operation or programming thereof.
Therefore, there is provided a signal line for the selection of an appropriate channel in the frequency band and a programming strobe line to indicate a programming mode. In case the programming strobe signal is supplied related data for the programming of the first programmable divider 208 and the second programmable divider 210 are supplied to the frequency generation unit 200 so as to select an appropriate channel in the single frequency band.
Still further, in case the frequency generation unit 200 should not output a frequency signal, it is set into the standby mode via the standby control line to reduce the amount of power consumed in the frequency generation unit 200.
After reprogramming of the frequency generation unit 200 a phase detector 212 detects a phase difference between the signals at the outputs of the first programmable divider 208 and the second programmable divider 210. Therefore, the phase detector 212 will drive the loop filter 214 until this phase difference vanishes. In other words, during the transition from the previously programmed output frequency to the newly programmed output frequency, there exists a transition time period wherein the phase detector drives the loop filter 214 such that the voltage-controlled oscillator 202 is tuned to the newly programmed operation frequency.
To this end, the phase detector comprises two parts, i.e. the actual phase difference detector and a charge pump (not shown).
As shown in FIG. 13, the phase detector unit works on the zero crossings of the input signals to the phase detector 212. One solution is to output a pulse with the same length as the time difference between the zero crossings of the input signals. In other words, this means that the output of the phase detector unit is proportional to the phase difference of the input signals supplied thereto.
Further, the phase detector unit has two different outputs, one for a positive phase difference and one for a negative phase difference. The respective output signals are supplied to a related charge pump that produces positive and negative current pulses with constant amplitude but different length which may then be processed through the loop filter 214.
In case the frequency generation unit 200 is locked to the frequency specified through the control unit 218, the phase detector 212 works in its linear region, as shown in FIG. 14. Before the frequency generation unit 200 is locked, the non-periodic behaviour of the phase detector 212 will force the frequency of the voltage-controlled oscillator 202 into the linear region of the phase detector 212 so that a locking of the frequency generation unit 200 is always guaranteed. For large initial frequency errors the phase detector operates in a frequency discriminator mode. Once the error is within the linear pull-in-range, it operates as a coherent phase detector, as shown in FIG. 14.
While the design illustrated with respect to FIG. 11 to FIG. 14 is adapted to, e.g., mobile phones being operated in a single frequency band this single band operation is no longer suitable for the increasing number of subscribers and the limited number of communication channels in existing cellular mobile networks. To the contrary, a combination of technical advantages being related to different frequency bands seems to be necessary, e.g., in particular through providing multiband cellular networks and multiband mobile phones being related thereto through combining, e.g., the GSM 900, GSM 1800 and PCS frequency bands, respectively.
However, a prerequisite is an effective frequency generation in a plurality of frequency bands and in particular an effective transition between these frequency bands within minimal time periods.
As shown in FIG. 15 wherein those parts being identical to those shown in FIG. 11 are denoted with same reference numerals, one approach is to use a plurality of voltage-controlled oscillators 220-1, . . . , 220-n, i.e. one voltage-controlled oscillator for each frequency band of the multiband frequency generation unit 222. The output of each voltage-controlled oscillator 220-1, . . . , 220-n is then coupled to the input of the second programmable divider via a coupling unit 224 achieving an appropriate supply of the output signals of the voltage-controlled oscillators 220-1, . . . , 220-n to the second programmable divider 210.
FIG. 16 shows a further approach to the multiband frequency generation that differs over the frequency generation unit as shown in FIG. 15 in that a loop filter 214-1, . . . , 214-n is provided for each of the voltage-controlled oscillators 220-1, . . . , 220-n. This leads to an additional advantage in that the transient behaviour for each single frequency band may be determined separately in compliance with frequency band specific requirements.
Therefore, using either approach shown in FIG. 15 or FIG. 16, it is not only necessary to switch between different channels in a single frequency band but also to switch between different bands in the frequency generation unit or equivalently to carry out frequency band jumps. This may require a re-programming of the first programmable divider 206 and the second programmable divider 210, and further to switch off the voltage-controlled oscillator in the old frequency band and to switch on the voltage-controlled oscillator in the new frequency band.
One example for such a transition would occur in a mobile phone that during a single GSM TDMA frame is active on three time slots. One is used for receiving, one for transmitting, and one for monitoring, respectively. While receive and transmit are usually carried out in the same frequency band, monitoring can either be in the same frequency band as receive and transmit or in a different frequency band. Therefore, the time between these slots determines the demand on the lock-in time in the frequency generation unit. In GSM mobile phone applications the most difficult transition occurs between monitoring and receive and must be carried out in the range of some hundred microseconds so that timing for this transition is highly critical.
However, as the approach outlined above with respect to FIG. 15 and FIG. 16 does not comprise any measures to coordinate the transition between the different frequency bands it may happen that the frequency synthesizer 216 is already programmed for the new frequency band although the voltage-controlled oscillator of the old frequency band is still active. Certainly, it is also possible that the situation is reversed, i.e. that the frequency synthesizer is still programmed for the old frequency band while the voltage-controlled oscillator for the new frequency band is already switched on.
In both cases, it is attempted to tune the currently active voltage-controlled oscillator to a frequency lying outside its specified frequency range such that the phase difference detected by the phase detector 212 gets excessively large. In other words, if a mismatch between the activated voltage-controlled oscillator and the programming of the programmable dividers exists in the frequency generation unit the steering output of the frequency synthesizer 216 goes to its tuning limit thereby losing its phase detector gain.
The result is a relatively long delay time, equivalently referred to as dead time, after the frequency synthesizer gets finally programmed to the suitable frequency band or the appropriate voltage-controlled oscillator gets switched on. Therefore, this mismatch leads to a significant impact on the lock-in time of the frequency generation unit as will be shown in the following with respect to FIG. 17 and FIG. 18.
According to the example shown in FIG. 17, a change of frequency bands is necessary from a first frequency band I to a second frequency band II. Here, as the voltage-controlled oscillator I gets switched off, the voltage-controlled oscillator II gets switched on, but for a short period of time the frequency synthesizer 216 is still programmed for the first frequency band I. This leads to a steering output of the loop filter 214 as shown in FIG. 17, where the different times may be classified as follows:
T1: voltage-controlled oscillator I gets switched off; voltage-controlled oscillator II gets switched on;
T2: the programmable dividers get programmed according to frequency band II, start of dead time;
T3: end of dead time, normal lock-in begins;
T4: the voltage-controlled oscillator II has finally reached the programmed frequency; and
Ti: the charge pump of the phase detector 212 loses its charge pump gain due to saturation.
Therefore, the example shown in FIG. 17 relates to the transition from a lower frequency band I to the higher frequency band II, e.g., from GSM 900 to GSM 1800 in a mobile phone. Further, the voltage-controlled oscillators are switched before the programming is finished. Therefore, the control circuit tries to tune the voltage-controlled oscillator II for the higher frequency band to the still prevailing programming for the lower frequency band. For this reason, the control voltage at the input of the second voltage-controlled oscillator II goes down to a minimum value between time T1 and time Ti. At time Ti the charge pump in the phase detector 212 reaches saturation and therefore loses its charge pump gain. This is the reason why at time T2 the locking-in does not start immediately. To the contrary, during the dead time between time T2 and time T3 it is necessary to bring the charge pump out of saturation and only then does the actual locking-in start at time T3.
A similar example illustrated in FIG. 18 occurs in case a transition is carried out from a higher frequency band II to a lower frequency band I and the programming of the programmable dividers in the frequency synthesizer 216 is only finished after the switching of the voltage-controlled oscillators. The times shown in FIG. 18 may be classified as follows:
T1: voltage-controlled oscillator II gets switched off and voltage-controlled oscillator I gets switched on;
T2: programming of programmable dividers for frequency band I is finished, start of dead time;
T3: end of dead time, begin of normal lock-in;
T4: the voltage-controlled oscillator I has finally reached the correct frequency;
Ti: charge pump in phase detector 212 is reaching saturation.
As shown in FIG. 18, according to this example the frequency generation unit at the start of the transition tries to tune the voltage-controlled oscillator for the lower frequency band I to the still prevailing programming for the higher, second frequency band so that the steering output for the first voltage-controlled oscillator for the lower frequency band is rising to the maximum value between time T1 and time Ti. Therefore, at time T2, when the programming for the lower frequency band I is finally finished, it is necessary to bring the charge pump in the phase detector out of saturation during the dead time between time T2 and time T3 before the actual locking-in begins at time T3 and ends at time T4.
It should be noted that the same effects as outlined above with respect to FIG. 17 and to FIG. 18 occur in case the programming in the frequency synthesizer is finished prior to the switching off the voltage-controlled oscillators.
In view of the above, the object of the invention is to avoid any dead time when switching between different frequency bands in a multiband frequency generation device.
According to the present invention, this object is achieved through a multiband frequency generation device, comprising a programmable multiband frequency synthesizer means to generate an output signal in at least two frequency bands, a control means adapted to operate and program the multiband frequency synthesizer means, respectively, wherein the control means sets the multiband frequency synthesizer means into a sleep mode during the programming thereof.
Therefore, the multiband frequency generation device according to the present invention avoids that a charge pump of the phase detector in the frequency generation unit runs into saturation during programming of the programmable multiband frequency synthesizer means. The reason for this is that the multiband frequency synthesizer means is deactivated or equivalently set into a sleep mode during the programming thereof such that no control operation steps are carried out during the programming. In consequence, the saturation of any charge pump in the phase detector outlined above may be completely avoided since a tuning of voltage-controlled oscillators is only carried out in case the frequency synthesizer is programmed appropriately. Therefore, the transition time between different frequency bands is reduced significantly thus increasing the range of possible applications with stringent timing requirements for the inventive multiband frequency generation device.
According to a preferred embodiment of the present invention the control means is adapted to initialize the sleeping mode slightly before the programming of the multiband frequency synthesizer means begins and to terminate the sleeping mode slightly after the programming of the multiband frequency synthesizer means terminates.
Therefore, as safety margins are provided at the beginning and the termination of the programming, any undefined operation conditions may be strictly avoided.
According to yet another preferred embodiment of the present invention, the multiband frequency synthesizer means comprises a voltage-controlled multiband oscillator to generate an output signal in each frequency band, and the control means comprises a sleep mode setting means adapted to maintain a steering signal for the control of the voltage-controlled multiband oscillator on a constant level during the sleep mode. Preferably, this object is achieved through a sleep mode setting means being adapted to set the power save control signal of the multiband frequency synthesizer means in order to maintain the steering signal for the control of the voltage-controlled multiband oscillator on a constant level during the sleep mode. This may for example be achieved by setting the output of the charge pump into a high impedance state. Also, the sleep mode may be set via programming instead of the issuance of a hardware signal.
Thus, this solution may be implemented without any hardware changes using the existing means for the control of the multiband frequency synthesizer means. Here, the standby mode usually provided for to save power during standby of the multiband frequency generation device is used to put this device into a sleep mode during programming thereof.
The same advantage arises in case the multiband frequency synthesizer means has a dedicated input for the control of the loop filter which may then alternatively be used to set the multiband frequency generation device into the sleep mode during programming.
According to yet another preferred embodiment, the sleep mode setting means comprises a programming strobe pulse spreading means adapted to receive a programming strobe pulse and to spread this pulse according to a predefined time period, and first switching means adapted to connect the power save control input terminal of the multiband frequency synthesizer means to ground during the predefined time period in response to the output signal of the programming strobe pulse spreading means.
This embodiment is advantageous in that the operation of the control unit in the multiband frequency generation device must not carry out the setting of the multiband frequency synthesizer means into the sleep mode. To the contrary, this is achieved automatically after issuance of a programming strobe signal which is available anyway.
According to yet another preferred embodiment, the sleep mode setting means comprises a first edge detecting means to detect a transition in a frequency band selection signal and second switching means adapted to connect the power save control input terminal of the multiband frequency synthesizer means to ground during the predefined time period in response to the output signal of the first edge detecting means.
Therefore, this implementation relies on a mode selection signal provided for the selection of the frequency band. Using this information, it is possible to avoid any activation of the sleep mode setting means at a time other than the transition between different frequency bands.
Finally, according to yet another preferred embodiment of the present invention, the sleep mode setting means comprises a second edge detecting means to detect an upward transition in a frequency band selection signal, third switching means adapted to connect the power save control input terminal of the multiband frequency synthesizer means to ground during the predefined time period in response to the output signal of the second edge detecting means, third edge detecting means adapted to detect a downward transition in a frequency band selection signal, and fourth switching means adapted to connect the power save control input terminal of the multiband frequency synthesizer means to ground during a predefined period of time in response to the output signal of the third edge detecting means. Preferably, the second and third edge detecting means are capacitors.
Therefore, the multiband frequency synthesizer means is set into the sleep mode only during programming thereof. Also, this setting can be achieved very cost-efficiently using capacitors to differentiate the frequency band selection signal to control a switch connecting the power save control input terminal to ground during programming of the multiband frequency synthesizer means.
Similar advantages as outlined above may be achieved through the inventive method for switching between different frequency bands in a multiband frequency generation device.